This invention is in the field of displays for computers and other electronic devices. More particularly, this invention relates to displays requiring an external light source for reading the display in low ambient light conditions.
Reducing power consumption is a desirable goal in the design of any electronic device. This goal becomes even more important when the electronic device is battery powered. In particular, designers of portable personal computers go to great lengths to reduce power consumption, as every reduction in consumption leads to greater battery life from a single battery charge and increased user convenience.
Many portable computers use a display that requires separate illumination. For example, liquid crystal displays (`LCD` s) do not generate their own light and therefore require separate illumination when operated in environments with low ambient light. Most often, a fluorescent lamp is used to illuminate the LCD from behind the LCD or from its side. Where such lights are used, they frequently account for from 30 to 40% of the power that the portable computer is using.
In common installations of LCDs and fluorescent lamps, the lamp is not mounted directly behind the display, where it would create a bright streak across the display, as well as necessitating a very thick display module. Instead, the lamp is mounted across one or more sides (an additional lamp being needed for each additional side illuminated) of the display and the light from the lamps is funneled into a plastic sheet behind the display. The plastic sheet is roughened in such a way that the light is scattered out of it to illuminate the display evenly. The plastic sheet is usually a few millimeters thick. In this description and in the industry, this plastic sheet is known as a light guide. Although the term "light guide" has also been used to describe other devices, including fiber-optic cables, its use herein will be restricted to describing the panel used to diffuse light across the back of the LCD.
Unfortunately, the coupling of light from the lamp into the display is not very efficient. Typically, only about 10% of the visible light generated by the lamp is actually coupled into the light guide.
A known lamp/display configuration is shown in FIG. 1. This standard mounting places lamp 10 at the edge of light guide 14. Reflector 20 is wrapped around lamp 10 to improve the coupling of light into plastic sheet 14. Lamp 10 additionally comprises plasma region 12 and phosphor layer 18. Although reflector 20 improves the light coupling somewhat, light that is emitted by plasma region 12 in an undesired direction must pass through phosphor layer 18 twice before there is any possibility of the light being coupled into light guide 14. Phosphor layer 18 is not transparent. Rather, it is moderately translucent. Therefore, most light that is initially misdirected is ultimately absorbed by phosphor layer 18 and converted to useless heat.
One effort to improve light coupling has involved making the bore of the fluorescent lamp smaller. This unfortunately increases the lamp's power density and consequently reduces the lamp's life. Also, small bore fluorescent lamps tend to have unstable plasma regions, leading to less reliable lamps.
Another known effort to modify a lamp to improve its coupling into a light guide involves leaving a small gap in the lamp's phosphor layer on the side of the lamp adjacent to the display. This arrangement is illustrated in FIG. 2, wherein phosphor layer 17 has a gap 9 left in it. Gap 9's purpose is to form an optical cavity from which photons can escape in only the desired direction. Unfortunately, this change in design results in little improvement. As phosphor layer 17 is not particularly reflective, a photon that is generated within the lamp is more likely to be absorbed by phosphor layer 17 (see lines B, E, F and G, FIG. 2) than it is to be reflected and escape through gap 9. Second, as in the lamp illustrated in FIG. 1, most light that escapes from the lamp through glass layer 16 and that is then reflected back through the glass into the lamp by reflector 20 is reabsorbed and converted into heat by phosphor layer 17 (see line D, FIG. 2).
Once light has found its way into light guide 14, light guide 14 acts to distribute the light evenly across the display. Often, scattering centers are made on or in the light guide to scatter the light out into display 22 (FIG. 1). Some of the scattered light escapes from the plastic sheet in the wrong direction, away from the display. To correct this, a common practice is to place a sheet of white plastic behind the plastic sheet to reflect this misdirected light back to the display.
Known lamp/display arrangements result in large losses of light, and consequently waste a great deal of power, in order to illuminate adequately the display to which they are coupled. An arrangement which could reduce these large losses and consequently reduce electrical power requirements would be a significant improvement.